Detecting anthropogenic carbon dioxide uptake and ocean

Biogeosciences, 9, 2509–2522, 2012
www.biogeosciences.net/9/2509/2012/
doi:10.5194/bg-9-2509-2012
© Author(s) 2012. CC Attribution 3.0 License.
Biogeosciences
Detecting anthropogenic carbon dioxide uptake and ocean
acidification in the North Atlantic Ocean
N. R. Bates1 , M. H. P. Best1 , K. Neely1 , R. Garley1 , A. G. Dickson2 , and R. J. Johnson1
1 Bermuda
2 Scripps
Institute of Ocean Sciences, Ferry Reach, Bermuda
Institution of Oceanography, University of California San Diego, La Jolla, California, USA
Correspondence to: N. R. Bates ([email protected])
Received: 14 December 2011 – Published in Biogeosciences Discuss.: 23 January 2012
Revised: 1 June 2012 – Accepted: 8 June 2012 – Published: 11 July 2012
Abstract. Fossil fuel use, cement manufacture and land-use
changes are the primary sources of anthropogenic carbon
dioxide (CO2 ) to the atmosphere, with the ocean absorbing approximately 30 % (Sabine et al., 2004). Ocean uptake
and chemical equilibration of anthropogenic CO2 with seawater results in a gradual reduction in seawater pH and saturation states () for calcium carbonate (CaCO3 ) minerals in
a process termed ocean acidification. Assessing the present
and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change
across ocean basins and at ocean time-series sites. Here, we
show the longest continuous record of ocean CO2 changes
and ocean acidification in the North Atlantic subtropical gyre
near Bermuda from 1983–2011. Dissolved inorganic carbon
(DIC) and partial pressure of CO2 (pCO2 ) increased in surface seawater by ∼40 µmol kg−1 and ∼50 µatm (∼20 %), respectively. Increasing Revelle factor (β) values imply that the
capacity of North Atlantic surface waters to absorb CO2 has
also diminished. As indicators of ocean acidification, seawater pH decreased by ∼0.05 (0.0017 yr−1 ) and  values by
∼7–8 %. Such data provide critically needed multi-decadal
information for assessing the North Atlantic Ocean CO2 sink
and the pH changes that determine marine ecosystem responses to ocean acidification.
1
Introduction
The emissions of anthropogenic CO2 to the atmosphere due
to fossil fuel use, cement manufacture and land-use changes
(Houghton, 2008) have increased rapidly over the last decade
(Friedlingstein et al., 2010). While anthropogenic CO2 accu-
mulates in the atmosphere, it is also taken up by the terrestrial
biosphere and oceans. The annual global ocean uptake is estimated at ∼1.4 to 2.5 Pg C yr−1 (e.g., Takahashi et al., 2002;
Manning and Keeling, 2006; Takahashi et al. 2009; McKinley et al., 2011; Pg = 1015 g), with annual rates of CO2 uptake
increasing with time (Le Quéré et al., 2009). The cumulative total global ocean uptake of anthropogenic CO2 since
pre-industrial times is estimated at ∼120–140 Pg C (Sabine
et al., 2004; Khatiwala et al., 2009). Ocean uptake of anthropogenic CO2 and seawater chemistry changes such as reduction in seawater pH and saturation states for calcium carbonate (CaCO3 ) minerals such as calcite (calcite ) and aragonite
(aragonite ) is termed ocean acidification (e.g., Calderia and
Wickett, 2003, 2005; Orr et al., 2005; Doney et al., 2009;
Feely et al., 2009), and has likely problematic consequences
for marine organisms and ecosystems that are, as yet, poorly
understood.
Assessments of rates of ocean CO2 uptake over multidecadal timeperiods, changes in the capacity of the ocean to
absorb CO2 and ocean acidification impacts are important
for predicting future climate change and marine ecosystem
responses (e.g., Fabry et al., 2009). These ocean carbon cycle and ocean acidification data provide critically needed observational tests of global coupled ocean-atmosphere models
and allow attribution of changes to both anthropogenic and
natural causes (e.g., Le Quéré et al., 2010). Repeat hydrographic sections provide a means of quantifying basinwide
ocean uptake of anthropogenic CO2 (e.g., Friis et al., 2005;
Brown et al., 2010; Wanninkhof et al., 2010). Alternatively,
higher frequency observations of the changes in seawater
chemistry due to uptake of anthropogenic CO2 have been
collected at a few ocean time-series near various islands,
Published by Copernicus Publications on behalf of the European Geosciences Union.
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
including near Bermuda (Bates, 2001, 2007; Bates and Peters, 2007), Hawaii (e.g., Dore et al., 2003; Brix et al., 2004;
Dore et al. 2009), the Canary Islands (Santana-Casiano et
al., 2007; Gonzalez-Davila et al., 2010) and Iceland (Olafsson et al., 2010). Here, we examine direct observations of the
seawater carbonate chemistry changes resulting from ocean
uptake of CO2 over the last 30 years in the subtropical gyre
of the North Atlantic Ocean near Bermuda from 1983–2011.
Seawater samples were collected from two time-series sites
near Bermuda (i.e., Bermuda Atlantic Time-series Study;
BATS; 31◦ 400 N, 64◦ 100 W (Michaels and Knap, 1998; Steinberg et al., 2001); and Hydrostation S, 32◦ 100 N, 64◦ 300 W;
Keeling, 1993). Samples for dissolved inorganic carbon (DIC
and total alkalinity (TA) were analysed (Bates et al., 1996a;
Dickson et al., 2007), multi-decadal checks on accuracy of
data are made and computations of other components of
the seawater carbonate system computed (Robbins et al.,
2010). These data provide directly observed indicators of
ocean acidification and include pH, carbonate ion concentration ([CO2−
3 ]) and the saturation state for calcium carbonate
(CaCO3 ) minerals. Finally, these data are combined with a
few earlier measurements in the North Atlantic Ocean (from
the Geochemical Ocean Section Study, GEOSECS, Kroopnick et al., 1972; and Transient Tracers in the Ocean, TTO
projects; Brewer et al., 1985; data corrected by Tanhua and
Wallace, 2005) extending these trends over the past 40 years.
2
2.1
Methods and materials
Seawater carbonate chemistry sampling at BATS
A time-series of observations of seawater carbonate chemistry observations in the upper ocean have been collected
in the subtropical gyre of the North Atlantic Ocean near
Bermuda since 1983 at the Bermuda Atlantic Time-series
Study (BATS) and Hydrostation S sites (Bates, 2007; Bates
and Peters, 2007). The combined ocean time-series data represent monthly water column sampling for DIC and total
alkalinity (TA) at BATS with analysis of samples at the
Bermuda Institute of Ocean Sciences (BIOS) using highly
precise and accurate coulometric and potentiometric techniques, respectively (Bates et al., 1996a,b; Bates, 2001). Additional surface samples were collected at Hydrostation S and
analysed for DIC and TA using manometric and potentiometric methods, respectively, at Scripps Institution of Oceanography (Keeling, 1993). Here, DIC is defined as (Dickson et
al., 2007):
2−
DIC = [CO∗2 ] + [HCO−
3 ] + [CO3 ]
(1)
where [CO2 *] represents the concentration of all unionized
carbon dioxide, whether present as H2 CO3 or as CO2 . The
total alkalinity of seawater (TA) is defined as:
Biogeosciences, 9, 2509–2522, 2012
2−
−
−
TA = [HCO−
3 ] + 2[CO3 ] + [B(OH)4 ] + [OH ]
+[HPO24 −] + 2[PO3−
4 ]+
−
+
[SiO(OH)−
3 ] + [HS ] + [NH3 ] + ... − [H ]
−[HSO−
4 ] − [HF] − [H3 PO4 ] − ...
(2)
2−
−
where [HCO−
3 ] + 2[CO3 ] + B(OH)4 are the principal components of seawater TA (Dickson et al., 2007).
2.2
Sampling frequency at BATS
The sampling frequency of the combined dataset from Hydrostation S and BATS was not uniform in time. In the
1980s, samples were collected 9–12 times a year, while since
1992, sampling increased to 14–15 times a year (Fig. 1).
The increase in sampling since the middle 1990s was due
to supplemental BATS bloom cruises (1 to 4 in number)
conducted during the January to April period in addition to
BATS core cruises. The increase in sampling frequency over
time weights the time-series to springtime when determining
non-seasonally aliased trends.
2.3
Sampling methods
At Hydrostation S, samples were collected into were collected into 1 litre Pyrex bottles with a change to 500 ml bottles in the early 1990’s, sealed, sealed with ground glass stoppers and then shipped to Scripps Institution of Oceanography
(SIO) for analysis (Keeling, 1993; Brix et al., 2004). Storage
time for samples before analysis ranged from a few months to
several years. Similar sampling protocols were established at
BIOS for sampling at the BATS (Bates et al., 1996a,b; Bates,
2001) but in the early 2000s, smaller Pyrex bottles (∼350 ml)
were used. Samples were typically analysed within a few
months of collection.
2.4
Analytical methods
Hydrostation S samples were analysed for DIC at SIO using manometric methods (Keeling, 1993) and potentiometric
titration methods were used for determination of TA (Keeling, 1993). Analytical precision for both DIC and TA at SIO
was typically <0.2 %. At BIOS, DIC was determined using
coulometric methods with a SOMMA system (Johnson et
al., 1993; Bates et al., 1996a; Dickson et al., 2007). During
the first two years of sampling, DIC samples were analysed
at WHOI (e.g., BATS cruise 1 to 21), and subsequently at
BIOS. DIC measurements were calibrated with known volumes of pure CO2 gas while certified reference materials
(CRM’s; Dickson et al., 2007) were routinely analysed each
day of analysis. Potentiometric titration methods were also
used for determination of TA at BIOS (Bates et al., 1996b).
At the beginning of the 1990s, a manual alkalinity titrator
was used for determination of TA. This was replaced by an
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
2.5
16
Sampling Frequency
14
12
10
8
*
6
4
2
2511
*
0
1985
1990
1995
2000
2005
2010
Year
Fig. 1. Sampling frequency or number of cruises each year when
Fig. 1carbonate chemistry and other parameters were collected.
seawater
From September 1988, only BATS cruises are shown with Hydrostation S cruise beforehand. The * symbol denotes years when the
full year was not sampled.
Fig. 2. Comparison of DIC analysed at BIOS and SIO. Differences
shown in µmol kg−1 .
automated VINDTA 2S (Versatile Instrument for the Determination of Titration Alkalinity) in the early 2000s. For both
manual and automated TA systems, 15–20 titration points
past the carbonic acid end point were determined for each
sample, with TA computed from these titration data using
nonlinear least squares methods (Dickson et al., 2007). Surface samples of Sargasso Sea water were also analysed each
day prior to sample analyses and CRMs were used routinely
to calibrate the TA measurements. Analytical precision for
both DIC and TA at BIOS was typically <0.2 % for several
thousand within bottle and between bottle replicate analyses
of more than 2000 samples.
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Comparison of replicate samples analysed at BIOS
and SIO
From 1989–2010, selected replicate surface DIC and TA
samples at BATS were analysed independently at BIOS and
Scripps using different analytical techniques. Replicate surface and 10 m depth samples were collected on BATS cruises
over a period of twenty years from 1990 to 2010. Comparison of DIC samples analysed independently at BIOS and SIO
indicate that the mean difference was 1.24 ± 3.35 µmol kg−1
(Fig. 2; SIO DIC values higher than BIOS values) with a
sample number of 373. Comparison of TA samples analysed
independently at BIOS and SIO indicate that the mean difference was 0.33 ± 5.02 µmol kg−1 (SIO TA values slightly
higher than BIOS values) with a sample number of 290.
Thus, over a period of twenty years, there is no evidence of
any systematic analytical biases that would influence the accuracy of data and trends.
2.6
Compilation of a combined BATS/Hydrostation
S record
Surface Hydrostation S data from September 1983 to
September 1988 and BATS data from October 1988 to
present (with exceptions) were combined to produce a continuous time-series. For BATS cruises 22–27, surface DIC
data from SIO (Keeling, 1993) was used as water-column
samples were not collected for the period July-December
1990. For BATS cruises 21–27 and 30–36, TA samples analysed at SIO were used (Keeling, 1993). For Hydrostation
28 S cruise 605698101, and BATS cruises 1–3, 4–20, 79A,
90A, 91A, 100A, 101A, 102A, 113A, 114A, 138, 185A
and 186A, TA was calculated from salinity (with an error
of ∼2.8 µmol kg−1 ; Bates et al., 1996a). Exclusion of these
samples from trend analysis did not change results significantly. The locations of Hydrostation S and BATS are separated in space by ∼50 km, but analysis of both Hydrostation
S and BATS data at SIO suggests that there is no statistical
difference between the two locations. Surface data are shown
here but no statistical difference was found if mean DIC and
TA were determined for different depth intervals in the mixed
layer (using a 0.5◦ C temperature criterion to define mixed
layer depth).
2.7
Computation of seawater carbonate chemistry
Seawater pCO2 , pH, [CO2−
3 ], mineral saturation states for
calcite (calcite ) and aragonite (aragonite ), and the Revelle factor (β) were computed from DIC, TA, temperature and salinity data using the programme CO2calc (Robbins et al., 2010). Carbonic acid dissociation constants (i.e.,
pK1 and pK2 ) of Mehrbach et al. (1973), as refit by Dickson and Millero (1987) were used for the computation, as
well as dissociation constants for HSO−
4 (Dickson, 1990).
GEOSECS and Transient Tracers in the Ocean (TTO) DIC,
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
A
Seawater pCO2
420
400
380
360
340
320
2005
B
40
Seawater pCO2 difference
440
20
0
-20
-40
2006
2007
2008
2009
2005
2006
2007
2008
2009
Fig.of3.computed seawater pCO2 (µatm) with pCO2 directly measured from the R/V Atlantic Explorer or Bermuda Testbed
Fig. 3. Comparison
Mooring (BTM). (A) time-series of computed seawaterpCO2 at BATS (black line) with directly measured pCO2 from the R/V Atlantic
Explorer and BTM (red symbols). The green symbols denote directly measured pCO2 corrected to the temperature of sampling at BATS. (B).
difference between computed seawaterpCO2 and directly measured pCO2 from the R/V Atlantic Explorer and BTM (blue symbols; µatm).
The cyan symbols denote directly measured pCO2 corrected to the temperature of sampling at BATS. Mean and 1 standard deviation are
shown in the figure.
29
TA, temperature and salinity data were taken from the
CDIAC site (http://cdiac.ornl.gov). We used the same dissociation constants to compute surface pCO2 , pH and aragonite
from GEOSECS and Transient Tracers in the Ocean (TTO)
DIC, TA, temperature and salinity data. GEOECS and TTO
data available through WAVES (Web-Accessible Visualisation and Extraction System) that allows access to discrete
data that are part of the Global Ocean Data Analysis Project
(GLODAP) and Carbon in the North Atlantic (CARINA)
databases. The strong caveat with regard to using GEOSECS
and TTO data is that these data have been adjusted (Tanhua
and Wallace, 2005) to account for potential biases associated
with the potentiometric determination of DIC at the time of
sampling. Thus, caution is urged when using these data for
comparison to time-series data at BATS/Hydrostation.
2.8
Computation of seawater carbonate chemistry
Both seawater pCO2 systems were calibrated with CO2 -inair standards calibrated at CMDL (NOAA).
At the time of sampling for DIC and TA at the BATS site
(i.e., time that Niskin sampler was tripped and filled), mean
observed R/V Atlantic Explorer and BTM seawater pCO2
were averaged over 1 h with the mid-point exactly contemporaneous with the time of sampling. Compared to the observed R/V Atlantic Explorer and BTM seawater pCO2 data,
computed pCO2 from DIC and TA was lower by a mean
of −4.7 ± 13.6 µatm. However, there were slight differences
in sampling temperatures and, thus, R/V Atlantic Explorer
and BTM seawater pCO2 data were corrected to the SST’s
at time of DIC/TA sampling at BATS. The mean difference
between temperature corrected seawater pCO2 computed at
BATS and directly observed from the R/V Atlantic Explorer
and BTM was small (−3.1 ± 10.8 µatm). No systematic bias
in the computation of seawater pCO2 was apparent.
2.9
The computation of seawater pCO2 was compared to direct measurements of pCO2 from the R/V Atlantic Explorer
and Bermuda Testbed Mooring (BTM) from 2005 to 2010
(Fig. 3). During this period, 42 direct comparisons were
made. Observed seawater pCO2 was collected from BIOS
ship R/V Atlantic Explorer (AE) at the BATS site (31◦ 430 N,
64◦ 100 W) using a pCO2 system calibrated with 4 CO2 in-air standards similar to previous methods used at BIOS
(Bates et al., 1998). Seawater pCO2 measurements were also
made using a pCO2 sensor attached to the Bermuda Testbed
Mooring (BTM; 31◦ 41.770 N, 64◦ 10.520 W; Dickey et al.,
2009, http://cdiac.ornl.gov). This system was calibrated with
one CO2 -in-air standard and deployed from 2005 to 2007.
Biogeosciences, 9, 2509–2522, 2012
Trend analysis and statistics
Trend analyses were conducted of the time-series of surface temperature and salinity, seawater carbonate chemistry
(DIC, TA, pCO2 , Revelle factor, β) and ocean acidification
indicators (pH, [CO2−
3 ], calcite and aragonite ). Here, trend
analysis of salinity normalised DIC (nDIC) and TA (nTA)
data were also made in order to account for local evaporation and precipitation changes. These data were normalised
to a salinity of 36.6, as this represents the mean salinity observed at the BATS site (Bates, 2007). Trend analysis was
performed with observed data (Table 1) and seasonally detrended data (Table 2). Regression statistics given were slope,
error, r 2 , p-value and n. Trends with p-values greater than
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
2513
Fig. 4. Mean monthly values for surface temperature and salinity (A), total alkalinity (B, µmol kg−1 ); DIC (C, µmol kg−1 ), seawaterpCO2
−1 for [CO2− ]), and 
and Revelle factor β (D, µatm), pH and [CO2−
calcite and aragonite (F), at BATS/Hydrostation S for the
3 ] (E, µmol kg
3
period 1983–2011. The standard deviation is shown by y error bars.
0.01 were deemed statistically not significant at the 99 %
confidence level.
2.10
Seasonal detrending of data
Trends analyses with observed data exhibits seasonal aliasing
due to sampling weighting to spring conditions. For example, sea surface temperature (SST) apparently cooled during
the 1983–2011 period at a rate of −0.075 decade−1 , but this
largely reflects a sampling bias that is weighted to springtime conditions. To account for seasonal weighting, the data
were also seasonally detrended. Seasonal detrending of the
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BATS/hydrostation S data was accomplished by binning data
into appropriate month, with mean values calculated from
two or more cruises conducted within a representative month
each year. This provides a uniform timestep of approximately
1 month (i.e., 365 or 366 days/12) throughout the timeseries, thereby removing any potential seasonal weighting especially to springtime conditions. The mean seasonality of all
parameters are shown in Fig. 4. Second, a mean and standard
deviation is then determined each month for the 1983–2011
(Fig. 4), and anomalies computed from monthly data minus
mean values. Trends and regression statistics for seasonally
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
Fig. 5. Long-term observations and trends of surface hydrography, seawater carbonate chemistry and ocean acidification indicators from 1983
to 2011 at the BATS (Bermuda Atlantic Time-series Study; 31◦ 400 N, 64◦ 100 W) and Hydrostation S (32◦ 100 N, 64◦ 300 W) sites located near
Bermuda in the NW Atlantic Ocean. Slopes and statistics of regressions are listed in Table 1. (A) Sea surface temperature (◦ C; black line)
and salinity (red line). (B) Surface total alkalinity (TA, µmol kg−1 , blue symbol) and salinity normalised TA (nTA; µmol kg−1 , cyan symbol).
(C) Surface dissolved inorganic carbon (DIC, µmol kg−1 , green symbol) and salinity normalised DIC (nDIC; µmol kg−1 , cyan symbol).
(D) Seawater pCO2 (µatm; purple symbol) and Revelle factor (β) (salmon symbol). (E) Surface seawater pH (orange symbol) and [CO2−
3 ]
−1
(µmol kg , yellow symbol) (F) Surface saturation state of calcite (calcite ) (purple symbol) and aragonite (calcite ) (light pink symbol).
detrended data are given in Table 2. Removing any potentially seasonal weighting allowed trend analysis of data that
had any non-temporal uniformity reduced as much as possible. The caveat to this approach and other approaches (e.g.,
using a 12 month harmonic fit to the data) is that energy may
be lost or gained is that variance may be lost.
Biogeosciences, 9, 2509–2522, 2012
3
3.1
Results
Seawater carbonate chemistry changes in
surface waters
The time-series of surface temperature and salinity, seawater carbonate chemistry (DIC, TA, pCO2 , Revelle factor,
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N. R. Bates et al.: Detecting anthropogenic carbon dioxide
2515
Fig. 6. Long-term anomalies and trends of surface hydrography, seawater carbonate chemistry and ocean acidification indicators from 1983
to 2011 at the BATS (Bermuda Atlantic Time-series Study; 31◦ 400 N, 64◦ 100 W) and Hydrostation S (32◦ 100 N, 64◦ 300 W) sites located near
Bermuda in the NW Atlantic Ocean. Slopes and statistics of regressions are listed in Table 1. (A) Sea surface temperature (◦ C; black line)
and salinity (red line). (B) Surface total alkalinity (TA, µmol kg−1 , blue symbol) and salinity normalised TA (nTA; µmol kg−1 , cyan symbol).
(C) Surface dissolved inorganic carbon (DIC, µmol kg−1 ,green symbol) and salinity normalised DIC (nDIC; µmol kg−1 , cyan symbol). (D)
Seawater pCO2 (µatm; purple symbol) and Revelle factor (β) (salmon symbol). (e) Surface seawater pH (orange symbol) and [CO2−
3 ]
(µmol kg−1 , yellow symbol) (f) Surface saturation state of calcite (calcite ) (purple symbol) and aragonite (calcite ) (light pink symbol).
β) and ocean acidification indicators (pH, [CO2−
3 ], calcite
and aragonite ) off Bermuda in the North Atlantic Ocean are
shown in Fig. 5. However, these data are subject to seasonal
weighting due to additional sampling in springtime and, thus,
seasonal detrended data are shown in Fig. 6. Trends and regression statistics for seasonally detrended data are given in
Table 2.
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3.2
Warming and salinity increases in surface waters
There is evidence for warming and increased salinity of surface waters of the North Atlantic subtropical gyre near Bermuda. Time series analysis of data
at BATS reveal significant long-term trends in the temperature and salinity of the surface ocean increasing at
Biogeosciences, 9, 2509–2522, 2012
Manuscript bg-2011-478. Bates, N.R., et al 2012, Biogeosciences. Revised text and figures
Seawater pCO2 attribution
2516
N. R. Bates et al.: Detecting anthropogenic carbon dioxide
ilar changes were observed previously at BATS (Bates,
2007) and attributed to the uptake of anthropogenic CO2
from the atmosphere (Gruber et al., 2002; Bates et al.,
2002). Similar changes in DIC over shorter timescales have
been observed elsewhere off Hawaii (Dore et al., 2003;
2009) and the Canary Islands (Santana-Casiano et al., 2007;
Gonzalez-Davila et al., 2010). TA increased slightly by
0.48 ± 0.07 µmol kg−1 yr−1 , attributed here to gradual increase in the salinity of surface subtropical gyre waters.
Salinity normalised TA (nTA) increased slightly at a rate
of 0.10 ±0.03 µmol kg−1 yr−1 , but since this trend was not
statistically significant, there was no definitive evidence that
nTA has changed during the last three decades (Table 2).
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
+3%
+8%
-33%
+122%
2
Salinity
3
Temp
4
TA
5
DIC
4
1
Total
Figure
7
Fig.
7. Long-term
trend in seawater pCO2 (purple) and its attribution to long-term changes in salinity, temperature, TA and DIC
(green). The relative attribution of change was computed using
CO2calc (Robbins et al., 2010) using mean temperature, salinity,
TA and DIC values observed at BATS. Mean values and trends taken
from Table 2.
rates of ∼0.011 ±0.006 ◦ C yr−1 (∼0.11◦ C decade−1 ) and
0.0054 ± 0.0001 yr−1 , respectively (Table 2; Fig. 6). However, it should be noted that the surface temperature trend is
not statistically significant at the 95 % level (p-value >0.05),
but similar temperature trends that are statistically significant
have been observed within the upper 400 m at BATS and
Hydrostation S over the last 55 years with temperature and
salinity increasing at rates of ∼0.01 ◦ C yr−1 and 0.002 yr−1 ,
respectively (Joyce et al., 1999). Similar long-term changes
in surface temperatures have been observed across the subtropical gyre of the North Atlantic Ocean (Grist et al., 2010)
indicating a system level change in the marine environment.
The observed increase in surface salinity near Bermuda is
also evident across the North Atlantic Ocean subtropical gyre
(Zhang et al., 2011), appears related to coordinated changes
in freshwater fluxes and modes of climate variability such as
the North Atlantic Oscillation (Hurrell and Deser, 2010).
3.3
Trends in seawater carbonate chemistry of
surface waters
33
Our observations near Bermuda also show multi-decadal
changes in seawater carbonate chemistry in response to
anthropogenic CO2 uptake by surface waters. Direct observations of DIC, computed seawater pCO2 and Revelle
factor (β) exhibit significant increases over the last three
decades (Fig. 6; Table 2). Surface DIC and salinity normalised DIC (i.e., nDIC) have increased by 1.39 ±0.06
and 1.08 ±0.06 µmol kg−1 yr−1 , respectively, a change of
nearly 40 µmol kg−1 , or ∼2 % from 1983 to 2011. SimBiogeosciences, 9, 2509–2522, 2012
4.1
Discussion
Changes in seawater pCO2 with time and
its attribution
Surface seawater pCO2 exhibited significant increases over the last 3 decades, increasing at a rate of
1.80 ± 0.09 µatm yr−1 and representing an increase of nearly
55 µatm or 20 % from 1983 to 2011 (Fig. 6; Table 2). The
increasing trend in seawater pCO2 can be attributed to
changes in DIC, TA, temperature and salinity (Fig. 7). The
trend in observed DIC would increase seawater pCO2 by
+122 %, but this effect is counteracted by a small increase
in TA that decreases pCO2 (−33 %). Trends in temperature (+8 %) and salinity (+3 %) have minor impacts on
calculated seawater pCO2 . Similar trends and attribution
of trends have been shown for observed pCO2 across the
North Atlantic in the seasonally stratified subtropical gyre
(Bates, 2011) where the increase in seawater pCO2 can be
attributed to non-temperature effects (i.e., sum of DIC, TA
and salinity changes). Over shorter timescales, similar trends
in surface seawater pCO2 have been observed elsewhere
off Hawaii (Dore et al., 2003; 2009) and the Canary Islands
(Santana-Casiano et al. 2007; Gonzalez-Davila et al., 2010).
4.2
Changes in the ocean CO2 sink?
These observations infer that the ocean CO2 sink in the
subtropical gyre has not changed significantly over the last
three decades. Similar to previous observations at BATS,
seawater pCO2 has increased at a comparable rate to atmospheric pCO2 (Bates, 2007; Takahashi et al., 2009; Table 2, 1.72 ± 0.01 µatm yr−1 ) from 1983 to present. These
trends indicate that the driving force for air-sea CO2 gas exchange (i.e., 1pCO2 ; pCO2 difference between atmosphere
and seawater) has not changed significantly over the last 3
decades. One might, therefore, think that the ocean CO2 sink
near Bermuda has not changed significantly over time. However, it should be noted that windspeed also contributes in
addition to 1pCO2 to the magnitude of air-sea CO2 gas exchange. There is some evidence that the annual CO2 sink
www.biogeosciences.net/9/2509/2012/
N. R. Bates et al.: Detecting anthropogenic carbon dioxide
2517
Table 1. Long-term trends (1983–2011) of surface hydrography and seawater carbonate chemistry with regression statistics (slope and
error, n, r 2 and p-value). This includes surface seawater temperature, salinity, DIC, nDIC, TA, nTA, atmospheric pCO2 (pCOatm
2 ), and
◦ 0
◦ 0
◦ 0
◦ 0
calculated seawater pCO2 (pCOsea
2 ) and Revelle factor (β) from the BATS (31 50 N, 64 10 W)/Hydrostation S (32 10 N, 66 30 W) sites
in the North Atlantic Ocean. Please see text for details on the CO2 -carbonic acid system computation and salinity normalisation procedures.
Observations of atmospheric pCO2 data come from two atmospheric monitoring sites (Terceira Island, Açores, September 1983–September
1988; Bermuda, August 1988–December 2010).
Period
Slope and std error
n
r2
p-value
09/1983–07/2011
09/1983–07/2011
−0.0075 ± 0.021 ◦ C yr−1
+0.0058 ± 0.0010 yr−1
362
362
0.00
0.08
0.72
< 0.01
−0.0016 ± 00022 yr−1
−0.64 ± 0.06 µmol kg−1 yr−1
−0.0152 ± 0.0016 yr−1
−0.0100 ± 0.0012 yr−1
358
358
358
358
0.13
0.24
0.21
0.16
< 0.01
< 0.01
< 0.01
< 0.01
+1.53 ± 0.12 µmol kg−1 yr−1
+1.20 ± 0.09 µmol kg−1 yr−1
+0.50 ± 0.07 µmol kg−1 yr−1
+0.12 ± 0.03 µmol kg−1 yr−1
+1.62 ± 0.21 µatm yr−1
+1.72 ± 0.01 µatm yr−1
+0.015 ± 0.0014 yr−1
361
361
361
361
358
1825
358
0.32
0.35
0.12
0.04
0.16
0.89
0.24
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
Parameter
Hydrography
Temperature
Salinity
Ocean acidification indicators
pH
CO2−
3
calcite
aragonite
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
Seawater carbonate chemistry
DIC
nDIC
TA
nTA
pCOsea
2
pCOatm
2
Revelle factor (β)
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–12/2010
09/1983–07/2011
has increased slightly due to higher annual windspeeds in
the 2000s observed near Bermuda (Bates, 2007), perhaps
in response to shift in the winter North Atlantic Oscillation
from positive to neutral/negative over the last 2 decades (Hurrell and Deser, 2010; Bates, 2012). Other studies have also
shown that with observations conducted over time periods
longer than 2 decades, seawater pCO2 has increased at approximately the same rate as the atmosphere (Bates et al.,
2002; McKinley et al., 2011) with longer term observations
smoothing out shorter term variability. Such studies suggest
that analysis of the CO2 sink or source status using data that
has a relatively short duration (<10 years) may not be sufficient to determine whether the North Atlantic Ocean CO2
sink has decreased over time (e.g., Schuster and Watson,
2007; Schuster et al., 2009; Watson et al., 2009). Such assessments may require a longer term view especially in light
of this and other studies (McKinley et al., 2011); and especially when deciphering anthropogenic secular trends from
natural variability imparted by such phenomena as the NAO
(Joyce et al., 1999; Gruber et al., 2002; Hurrell and Deser,
2010) and Atlantic Multidecadal Oscillation (McKinley et
al., 2011). For example, in the last couple of years (2010–
2011; Figs. 5 and 6) there was a lowering of surface seawater
pCO2 which appears associated with a change in the NAO
state (Bates, 2012). Thus, undertaking trend analysis over the
last decade would show a much reduced increase in seawater
www.biogeosciences.net/9/2509/2012/
pCO2 compared to the atmosphere with the inference that
the CO2 sink in this region had increased. This may well be
the case over the short-term, but over several decades it appears that 1pCO2 values have not changed significantly and,
thus, there is little evidence of any substantial change in the
CO2 sink in the subtropical gyre of the North Atlantic near
Bermuda.
In addition, over the 1983–2011 period, the Revelle factor
(β) has increased at a rate of 0.0137 ± 0.0008 yr−1 or ∼0.41
over the last 3 decades. This indicates that the buffer capacity
of subtropical gyre surface waters to absorb CO2 has gradually reduced over time, confirming previous model studies
that predict an increasing trend in β at BATS and for the
North Atlantic Ocean in response to the ocean uptake of anthropogenic CO2 from the atmosphere (Thomas et al., 2007).
4.3
The signal of ocean acidification in the
North Atlantic Ocean
The BATS/Hydrostation S time-series data allow direct detection of the signal of ocean acidification in surface waters of the North Atlantic. The uptake of anthropogenic CO2
from the atmosphere by the ocean changes seawater chemistry through chemical equilibrium of CO2 with seawater.
Dissolved CO2 forms a weak acid and the pH and [CO2−
3 ]
decrease as seawater absorbs CO2 , a process termed ocean
Biogeosciences, 9, 2509–2522, 2012
2518
N. R. Bates et al.: Detecting anthropogenic carbon dioxide
Table 2. Seasonally detrended long-term trends (1983–2011) of surface hydrography and seawater carbonate chemistry with regression
statistics (slope and error, n, r 2 and p-value). This table includes trend analysis of surface hydrography including temperature and salinity,
sea
seawater carbonate chemistry (i.e., DIC, nDIC, TA, nTA, atmospheric pCO2 (pCOatm
2 ), and calculated seawater pCO2 (pCO2 ) and Revelle
factor (β) anomalies) and indicators of seawater carbonate chemistry changes (e.g., pH, [CO2−
3 ], and mineral saturation states of calcite
(calcite ) and aragonite (arag ) from the BATS (31◦ 400 N, 64◦ 100 W) and Hydrostation S (32◦ 100 N, 64◦ 300 W) sites in the North Atlantic
Ocean. Please see text for details on the seawater carbonate chemistry computation and salinity normalisation procedures. Observations of
atmospheric pCO2 data come from two atmospheric monitoring sites (Terceira Island, Açores, September 1983–September 1988; Bermuda,
August 1988–December 2010). Please see text for details on the methods for removing seasonal trends.
Parameter
Period
slope and std error
(and units if applicable)
n
r2
p-value
+0.33 ◦ C
+0.162
+41.7 µmol kg−1
+32.4 µmol kg−1
+14.6 µmol kg−1
+3.0 µmol kg−1
+54.4 µatm
+51.6 µatm
+0.41
+0.011 ± 0.006 ◦ C yr−1
+0.0054 ± 0.0001
+1.39 ± 0.06 µmol kg−1 yr−1
+1.08 ± 0.05 µmol kg−1 yr−1
+0.48 ± 0.07 µmol kg−1 yr−1
+0.10 ± 0.03 µmol kg−1 yr−1
+1.80 ± 0.09 µatm yr−1
+1.72 ± 0.01 µatm yr−1
+0.0137 ± 0.0008
320
320
319
319
319
319
319
1825
319
0.01
0.09
0.63
0.61
0.14
0.03
0.53
0.89
0.50
0.06
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
< 0.01
−0.0501
−17.5 µmol kg−1
−0.246
−0.391
−0.00174 ± 0.0001
−0.58 ± 0.04 µmol kg−1 yr−1
−0.0091 ± 0.0006
−0.0141 ± 0.0009
319
319
319
319
0.50
0.41
0.39
0.42
< 0.01
< 0.01
< 0.01
< 0.01
30 yr change
(and units if applicable)
Hydrography and seawater carbonate chemistry
Temperature
Salinity
DIC
nDIC
TA
nTA
pCOsea
2
pCOatm
2
Revelle
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–12/2010
09/1983–07/2011
Ocean acidification indicators
pH
CO2−
3
aragonite
calcite
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
09/1983–07/2011
acidification (Caldeira and Wickett, 2003, 2005). The effects of ocean acidification are potentially far-reaching in the
global ocean, particularly for organisms that secrete CaCO3
skeletons, tests or shells and for marine ecosystems where
calcification and pH controls on biogeochemical processes
are important factors (e.g., Fabry et al., 2009). Relevant indicators of ocean acidification include seawater pH, but also
[CO2−
3 ] and, thus, saturation states for CaCO3 minerals (i.e.,
calcite and aragonite ). The typical pH range of surface waters is between 7.8 and 8.4 in the open ocean and the ocean
remains mildly alkaline at present. At BATS, the mean pH is
8.094 with a range of 8.00–8.18 (Fig. 5).
The BATS site near Bermuda constitutes the longest
time-series record of ocean acidification anywhere in the
global ocean (Fig. 6; Bates, 2007). Trend analysis shows
that the primary indicator of ocean acidification, seawater pH, has decreased at a rate of −0.0017 ± 0.0001 yr−1 ,
a total decline in seawater pH of ∼0.05 over the past 3
decades (Fig. 6; Table 2). This represents a ∼12 % increase in hydrogen ion concentration since 1983. Other indicators of ocean acidification at BATS such as [CO2−
3 ],
calcite and aragonite have also decreased at a rate of
−0.58 ±0.041 µmol kg−1 yr−1 , −0.0141 ± 0.0009 yr−1 , and
−0.0091 ± 0.0006 yr−1 , respectively (Table 2). Trend analysis at BATS and other published trends for three other ocean
Biogeosciences, 9, 2509–2522, 2012
time-series sites indicate that surface seawater pH has decreased at a rate of −0.0014 to −0.0019 yr−1 (e.g., Bates
and Peters, 2007; Bates, 2007; Dore et al., 2009; GonzalezDavila et al., 2010; Byrne et al., 2010).
5
Conclusions
The long term ocean observations near Bermuda constitute the longest running time-series of changes in seawater carbonate chemistry due to the uptake of anthropogenic
CO2 and resulting ocean acidification impacts. Such records
and those from other time-series sites (Dore et al., 2009;
Gonzalez-Davila et al., 2010; Olafsson et al., 2010) provide
critically needed data showing that such changes are due
to anthropogenic CO2 release and absorption by the global
ocean and to test ocean-atmosphere models (e.g., Bates,
2007; McKinley et al., 2011) that are used for predictive
climate-change purposes. Before regular sampling began at
the Hydrostation S in 1983 and later at the BATS site in 1988,
there was occasional sampling in the North Atlantic subtropical gyre near Bermuda through the GEOSECS and Transient
Tracers in the Ocean (TTO) projects. Here, we compute surface pCO2 , pH and aragonite from surface GEOSECS and
TTO DIC and total alkalinity data (Kroopnick et al., 1972;
Brewer et al., 1985). Combined with BATS/Hydrostation S
www.biogeosciences.net/9/2509/2012/
N. R. Bates et al.: Detecting
anthropogenic
carbon
Manuscript
bg-2011-478. Bates,
N.R., et aldioxide
2012, Biogeosciences. Revised text and figures
2519
Date
1970
420
1975
1980
1985
1990
1995
2000
2005
2010
1985
1990
1995
2000
2005
2010
A
400
pCO2
380
360
340
320
300
GEOSECS
TTO
GEOSECS
TTO
280
8.20
B
pH
8.15
8.10
8.05
8.00
4.2
C
aragonite
4.0
3.8
3.6
GEOSECS
TTO
3.4
3.2
1970
1975
1980
Figure 8
Fig. 8. Time-series of atmospheric and ocean pCO2 , pH and aragonite saturation states. (A) time-series of atmospheric pCO2 (ppm) from
Mauna Loa, Hawaii (red line), and Bermuda (pink symbol), and surface ocean seawater pCO2 (µatm) at the Bermuda Atlantic Time-series
Study (BATS) site off Bermuda. Observed (grey) and seasonally detrended (purple) surface ocean seawater pCO2 levels are shown. Earlier
seawater data from the GEOSECS (stations 67◦ 58◦ W, 33◦ 590 N; 56◦ 330 W, 33◦ 200 N) and TTO (stations 67◦ 21◦ W, 34◦ 410 N; 61◦ 20◦ W,
34◦ 420 N; 56◦ 110 W, 32◦ 080 N) expeditions in the North Atlantic Ocean are also shown in this and following panels. (B) time-series of
surface ocean seawater pH at the BATS site off Bermuda. Observed (grey) and seasonally detrended (orange) seawater pH are shown. (C).
34
time-series of surface ocean aragonite saturation state (aragonite ) for calcium carbonate at the BATS site off Bermuda. Observed (purple)
and seasonally detrended (purple line) seawater aragonite are shown.
data, this extends the time-series records of computed surface pCO2 , pH and aragonite back to the early 1970s (Fig. 8).
These data were sampled within 200 km of Bermuda (but
not at BATS/Hydrostation) and both surface pCO2 , pH and
aragonite data from GEOSECS and TTO fall close to the regression lines for these parameters (from Table 1, Fig. 2).
The trend line is hindcast for seawater carbonate chemistry in
Fig. 8 into the 1970s, but the actual trend in the 1970s would
likely follow the nonlinear trend of the atmospheric pCO2
record. A final caveat in showing the GEOSECS/TTO data
is that these data are adjusted (Tanhua and Wallace, 2005) to
account for measurement biases and not adjusted for seasonality, so some caution is urged in comparing these data with
those from BATS/Hydrostation S. Notwithstanding these issues, the trends established at BATS/Hydrostation S appear
to extend back to the early 1970s, constituting a nearly con-
www.biogeosciences.net/9/2509/2012/
tinuous 40 year record of changing seawater carbonate chemistry and ocean acidification indicators.
Acknowledgements. The authors wish to express their deep appreciation to many following individuals: Anthony H. Knap (principal
investigator) and other present and past BATS investigators,
Anthony F. Michaels, Dennis A. Hansell, Deborah K. Steinberg,
Craig A. Carlson and Michael W. Lomas. We are deeply in debt
to the pioneering work of the late Dave Keeling (d. 2005). Past
and present analysts for seawater carbonate chemistry at BIOS
for the BATS project include Frances A. Howse, Julian Mitchell,
Brett Purinton with data QC/QA by Christine Pequignet and
Marlene Jeffries, and at the Scripps Institution of Oceanography,
Peter Guenther, Timothy Lueker and Guy Emanuele. Many BIOS
scientists and technicians have spent dedicated time at sea sampling
at BATS and Hydrostation S, including: Timothy J. Jickells,
Rachel Dow, Kandace Binkley, Ann Close, Kjell Gundersen, Jens
Biogeosciences, 9, 2509–2522, 2012
2520
Sorensen, Elizabeth Caporelli, Fred Bahr, Steven J. Bell, Patrick
Hyder, Vivienne Lochhead, Paul Lethaby, Marta Sanderson, Megan
Roadman, Debra A. Lomas, Mary-Margaret Murphy, Sybille
Pluvinage, Lilia M. Jackman, Andreas J. Andersson, Matthew A.
Tiahlo, Jonathan D. Whitefield, Kevin Lew, Dafydd (Gwyn) Evans,
Elise van Meerssche, James Sadler, Sam Monk and Matthew
Wilkinson. The captains and crews of the R/V Weatherbird, R/V
Weatherbird II, R/V Cape Hatteras and R/V Atlantic Explorer. The
National Science Foundation is thanked for its support of both
Hydrostation S and BATS projects.
Edited by: G. Herndl
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